telecommunications switching systems 5
TRANSCRIPT
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Wideband Transmission Media
Coaxial Cable
Microwave Radio
Frequency Diversity
Space Diversity
Satellites
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Co-axial cable
Radio-grade flexiblecoaxial cable.
A: outer plastic sheath
B: copper screen
C: inner dielectricinsulator
D: copper core
Coaxial MUX
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Microwave Radio
Freq Diversity
The signal is transferred using severalfrequency channels or spread over a widespectrum that is affected by frequency-selective fading.
Middle 20th century microwave radio relaylines often used several regular wideband
radio channels, and one protectionchannel for automatic use by any fadedchannel.
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Space Diversity
The signal is transferred over several differentpropagation paths. In the case of wired transmission, thiscan be achieved by transmitting via multiple wires. In thecase of wireless transmission, it can be achieved byantenna diversity using multiple transmitter antennas(transmit diversity) and/or multiple receiving antennas(diversity reception).
Transmission Impairments
Signal Attenuation
Interference
Noise
Distortion
Echoes and Singing
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Signal Attenuation
Subjective listening tests have shown thatthe preferred acoustic to acoustic loss in atelephone connection should be inneighborhood of 8 dB.
Signal Attenuation
A typical local call had only 0.6 dB moreloss than ideal.
The average analog toll connection had anadditional 6.7 dB loss.
The standard deviation of loss in toll
connections was 4 dB. Trunks within the toll network used
amplifiers to offset transmission losses.
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Interference
Interference is more structured than noisesince it arises as unwanted coupling from
just a few signals in the network.
Interference if intelligible, is referred to ascrosstalk.
Sources of crosstalk
Coupling between wire pairs in cable
Inadequate filtering or carrier offsets inolder FDM equipment, and the effects ofnon-linear components on FDM signals.
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Forms of Crosstalk
Near-end Crosstalk (NEXT)
Far-end Crosstalk (FEXT)
Which form of crosstalk is more troublesome?
Noise
White Noise : easy to analyze, easy to find,arises as thermal noise, truly random,
uncorrelated, quantified in terms of average
power.
Impulse noise: occur from switching transients inolder equipment, measured in terms of impulses
per second.
Impulse noise is usually of less concern to voice
quality than background white noise. Impulse noise is a problem in data
communication.
Quantization Noise
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Weighting Curves
Disturbances at some frequencies withinthe passband of a voice signal aresubjectively more annoying than others.
Thus, more useful measurements of noiseor interference power in a speech networktake into account the subjective effects ofthe noise as well as the power level.
C-message and Psophometric Weighting
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Weighting Curves
These curves essentially represent filtersthat weight the frequency spectrum ofnoise according to its annoyance effect toa listener.
C-message is a north American standardwhile psophometric weighting is aEuropean (ITU-T) Standard.
Relationships between various noisemeasurements
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Distortion
Due to Internal characteristics of channel,deterministic in nature.
Can be compensated or controlled, once known.
Some distortions arises from non-linearities (e.g.saturated VF amplifiers) in the network while
other are linear in nature.
Amplitude and Phase distortion.
Amplitude Distortion
Attenuation of some frequencies in thevoice spectrum more than others.
Introduced by spectrum limiting filters inFDM eqpt.
Loading coils eliminate Amp distortion.
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IL vs fof typical toll connection
Phase Distortion
Delay characteristics of transmission medium.
Individual frequency components experience differentdelays.
Delay of Individual Freq. component is referred to as itsenvelope delay
Uniform envelope delay linear phase systems.
Any deviation from a linear phase characteristic is
referred to as phase distortion. Perceptual effects of phase distortion to a voice signal
are small. Thus requires minimal attention.
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Envelope delay and phase response
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In addition to amp and phase
distortion, other frequency relateddistortions are:
frequency offsets,jitter, phase hits and signal dropouts.
Echoes & Singing
Transmitted energy coupled to return path.
Most common cause impedancemismatch at a 2-to-4-wire hybrid.
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Echoes & Singing
Talker echo 1st reflection
Listener echo 2nd reflection
Singing repeated coupling to forwardpath leads to oscillations (if loop gain at afrequency 1).
Echoes & Singing
Talker echo is usually most noticeable andtroublesome.
The degree of echo annoyance isdependent on magnitude of returning
signal and the amount of delay involved.
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Echo Suppressor and Canceller
Power Levels
ITU-T zero-relative level point
North American zero-transmission-levelpoint (0-TLP).
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TLP
Transmission Level Point, or TLP, issimply one way of representing gain orloss in a channel in dB.
loss
0 TLP -3 TLP3 dB
loss
-16 TLP -19 TLP3 dB
0 TLP, Zero Transmission Level Point Reference Point
1.5 dB 3 dB 4.5 dB dBm
0 TLP -1.5 TLP +1.5 TLP -3 TLP
loss lossgain
1.5 dB 3 dB 4.5 dB dBm
0 TLP -1.5 TLP +1.5 TLP -3 TLP
loss lossgain
-10 dBm0-10 dBm
-10 dBm0-11.5 dBm
-10 dBm0-8.5 dBm
-10 dBm0-13 dBm
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TLP = dBm dBm0
dBm0 means power at 0 TLP.
TLP does not describe the (absolute)power present at a point.
To know the power present at a givenTLP, it is necessary to know the power
present at some other TLP in the channel.
Example 1: If a signal is specified as -13 dBm0 at aparticular point and -6 dBm is measured at that point,the TLP is
TLP = -6 dBm ( - 13 dBm0) = +7 TLP
Example 2: If a signal is -13 dBm at the 0 TLP, thenat the +7 TLP the signal level in dBm is
(+7 TLP) + ( - 13 dBm0) = -6 dBm
Example 3: A -13 dBm0 signal measured at -16 TLP is
(-16 TLP) + ( - 13 dBm0) = -29 dBm
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TLP
TLP applies to noise levels as well assignal levels.
When referred to the 0 TLP, noise is givenin dBrnC0.
TLP = dBrnC dBrnC0
Example 4: A 71 dBrnC0 signal measured at -3 TLP is
(-3 TLP) + ( 71 dBrnC0) = -68 dBrnC
Example 5 : A 71 dBrnC0 signal measured at +7 TLP is
(+7 TLP) + ( 71 dBrnC0)= 78 dBrnC
0/0 TLP means a circuit has zero gain (loss)-16/+7 TLP > Rx/Tx TLP
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Example 1.1 (Text book)